Optical recording medium with aligned prepit portion

ABSTRACT

An optical recording medium having an aligned prepit portion straddled on a plurality of tracks in a radial direction. The prepit portion includes first and second prepit portions divided in a track direction with a prepit of the respective first and second prepit portions being arranged on a boundary of the respective tracks. The first prepit portion has an address information prepit and a synchronous information prepit and the second prepit has a synchronous information prepit. The prepit of the first prepit portion and the prepit of the second prepit portion are arranged at every two-track pitch in the radial direction and the prepit of the first prepit portion and the prepit of the second prepit portion are arranged with one track displaced in the radial direction. A gap is provided between the first and second prepit portions.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical recording medium andmore particularly to a high-density optical recording medium having atrack width smaller than an optical spot diameter.

[0002] An example of a medium for performing high-density (narrow track)recording is disclosed in, for example, JP-A-6-176404. According to thisexample, in an optical recording medium having grooves and lands whichare formed on a substrate and information recording areas which areformed in association with both the groove and the land, prepits aredisposed on a virtual extension line of the boundary between a grooveand a land. In particular, the prepits are located on only one side ofany specific position of the center line of each groove.

[0003] With this construction, recording information is formed on boththe groove and the land, the prepits have charge of address datarepresentative of the recording areas and one prepit is used in commonto a pair of adjacent groove and land to provide address data therefor.When the technique as above is applied to, for example, a phase changerecording medium or a magneto-optical recording medium, interference ofinformation (crosstalk) between adjacent lands or grooves due to theoptical interference effect within an optical spot can be prevented,thereby permitting narrowing of track.

[0004] On the other hand, in the prepit area free from the opticalinterference effect, the address data can be common to the paired grooveand land and the effective track pitch can be increased to reducecrosstalk.

[0005] In the example of JP-A-6-176404, however, the disposition of theprepit area is offset on one side of the center line of the groove orland, so that when an optical spot is caused to track a groove or aland, a tracking error (tracking offset) increases, making it difficultto perform high-density recording in which the track pitch is narrowed.

SUMMARY OF THE INVENTION

[0006] The present invention achieves elimination of the above problemsand it is a first object of the present invention to provide an opticalrecording medium which can suppress the tracking offset to a value orlevel which is sufficiently low for the practical use and permitefficient disposition of address data even when recording is effected onboth the groove and the land.

[0007] A second object of the present invention is to provide ahigh-density optical recording medium which can ensure simple masteringand easy replica preparation and can permit decoding even when a readouterror takes place.

[0008] To accomplish the above first object, the following expedientsare employed.

[0009] (1) Grooves and lands are formed on a substrate of a recordingmedium, information recording areas are formed in association with boththe groove and the land, and prepits are disposed on a virtual extensionline of the boundary between a groove and a land.

[0010] The disposition of prepits satisfies the following requirements(a) to (c) at the same time.

[0011] (a) Prepits are located on both sides of a virtual extension ofthe center line of a groove;

[0012] (b) Prepits are located on both sides of a virtual extension ofthe center line of a land;

[0013] (c) Prepits are not located on the both sides of any specificposition of the center line of a groove; and

[0014] (d) Prepits are not located on the both sides of any specificposition of a land.

[0015] With this construction, the arrangement of prepits is not offseton either one side of a virtual extension of the center line of thegroove or the land to ensure that tracking offset hardly occurs and theprepits do not exist on both sides of any specific position of thecenter line of the groove or the land to prevent interference of prepitinformation between adjacent tracks from taking place within areproduced spot so as to permit high-density narrow track recording.

[0016] (2) When prepits are disposed in the circumferential directionsuch that those on one side of a groove are not discriminative fromthose on the other side or those on one side of a land are notdiscriminative from those on the other side, at least consecutive twodispositions of prepits associated with the groove or the land are madeto be different from each other to provide the same disposition ofprepits periodically every two dispositions.

[0017] As the other option,

[0018] (3) A groove associated with at least one pair of pits disposedon both sides of the center line of the groove in a prepit area and anadjacent groove not associated with pits disposed on both sides of thecenter line of this groove within the prepit area are disposedalternately in the radial direction.

[0019] Through this, by merely reproducing the pits, prepits associatedwith the groove can be discriminated from those associated with the landto improve reliability of information recording reproduction.

[0020] (4) Either one of synchronous information and address data isrepresented by prepits disposed on either one of the both sides of agroove.

[0021] As the other option,

[0022] (5) Only one of synchronous information and address data isrepresented by prepits arranged on one side of a groove and both thesynchronous information and the address data are represented by prepitsarranged on the other of the both side of the groove.

[0023] Through this, address data can be reproduced under accuratesynchronization. In addition, since the phase margin between prepits onthe both sides can be extended, fabrication of a recording medium can befacilitated.

[0024] (6) The groove and the prepit have the same depth which is 70 nmor less. More preferably, the depth is 40 nm or more and 60 nm or less.

[0025] With this construction, an advantage of suitable crosstalkcancellation can be obtained between the groove and the land and besidesan excellent tracking servo signal can be obtained. Formation andfabrication of the recording medium can be facilitated. With the groovedepth exceeding 70 nm, the formation of the groove is difficult toachieve. When the groove depth is about 50 nm, the tracking servo ismaximized and with the groove depth being about 50±10 nm, substantiallythe same effect can be attained.

[0026] (7) The groove and the land have substantially the same widthwhich is between 0.3 μm and 0.75 μm.

[0027] With this construction, excellent tracking is compatible withhigh-density recording. If the groove and land have a width of notgreater than 0.3 μm, then two of the groove and land will be confinedwithin one optical spot and an excellent tracking signal cannot beobtained. On the other hand, if the groove and land have a widthexceeding 0.75 μm, then effective high-density recording cannot bepermitted.

[0028] (8) of prepits, the smallest one has a diameter which is smallerthan a width of each of the groove and the land. More preferably, thediameter is in the range from 0.25 μm to 0.55 μm.

[0029] Through this, an excellent prepit signal can be obtained withoutcrosstalk. With the diameter being not greater than 0.25 μm, the prepitsignal decreases in the extreme and with the diameter exceeding 0.55 μm,crosstalk is generated.

[0030] In the present invention, prepits are arranged on the both sidesof a virtual extension line of the center line of a groove or a land inthe optical spot scanning direction. Consequently, offset is decreasedto make the tracking offset hardly occur and the prepits do not exist onthe both sides of any specific position of the center line of the grooveor the land to ensure that interference of prepit information betweenadjacent tracks within a reproduced spot can be prevented andhigh-density narrow track recording can be permitted.

[0031] Further, even in the presence of tracking offset, the amount oftracking offset can be detected accurately by comparing amplitudes ofsignals representative of prepits on the both sides. Accordingly, byfeedback-controlling the information indicative of a comparison resultto a scanning unit, the tracking offset can be suppressed.

[0032] At a portion between a groove and a prepit area, between a landand a prepit area or between prepit areas, a gap takes place when aprepit train on a virtual extension line of the boundary between agroove and a land shifts to a prepit train on an adjacent virtualextension line. The aforementioned JP-A-6-176404, however, does not takethe gap into consideration. Accordingly, in the absence of the gap orwith the gap being very short, mastering of the substrate cannot beproceeded with by one-beam cutting and requires two-beam cutting.Further, during replica preparation, injection must be applied to asteep pattern, leading to a decrease in yield. In addition, duringreproduction of signals, tolerance to distortion of the reproduced spotand the tracking offset is decreased and a readout error is liable tooccur.

[0033] To accomplish the second object, the following expedients areemployed.

[0034] (1) Grooves and lands are formed on a substrate and prepits arearranged on a virtual extension of the boundary between a groove and aland. In particular, the prepits are disposed on both sides of anextension of the center line of a groove or a land and therefore, theoptical axis of a laser beam must be moved during cutting. Anacoustic-optical deflector (AOD) is used to change the optical axis. Butit takes a time for the AOD to cause the optical axis to reach a desiredoptical axis position after transmitting a signal for optical axischange and when a modulated laser beam is irradiated along the intactoptical axis, pits are formed obliquely on the substrate. Accordingly,no pattern is formatted between the groove or the land and thesucceeding prepits to provide a gap and an acoustic-optical modulator(AOM) is cut off corresponding to the gap to prevent laser irradiationand pit drawing. Thus, the substrate can be fabricated with a simplecutting machine. In addition, since a number of unevennesses are notformed on a narrow area on the substrate, the yield during preparationof replica can be increased.

[0035] (2) In the disposition in which prepits are arranged on a virtualextension of the boundary between a groove and a land, when thedisposition of a prepit train on one side of a virtual extension of theboundary between the groove and the land is exchanged with thedisposition of a prepit train on the other side or vice versa, thetrailing edges of prepit trains on the respective one sides are alignedwith each other in the radial direction of the substrate. The succeedingpit trains are spaced from those trailing edge positions in thecircumferential direction or the recording/reproducing direction and thetrailing edges of the succeeding pit trains are aligned with each othersimilarly. When the formed gap meets the recording rule, the substrateas a whole can be formatted conveniently and portions devoid of pits canbe collected at a specified area on the substrate, thereby solvingproblems involved in cutting and replica preparation for reasonsdescribed previously.

[0036] (3) Radially adjacent pit trains each having only originalinformation pits cannot be aligned with each other at the trailing edgein the radial direction. Accordingly, new pits are added to ensure thealignment of the trailing edges in the radial direction while observingthe rule during recording.

[0037] (4) In the shift of the disposition of a pit train from one sideto the other as described in the above (2), leading edges of pits in thedisposition on the other side can be aligned in the radial direction tosolve the problems involved in cutting and replica preparation for thesame reasons set forth in the (2). In particular, from the standpoint ofsignal reproduction, a synchronous signal is allotted to pit informationimmediately after the shift of the pit train so that decision of achannel bit at the specified position may be thought much of, therebyensuring that the tolerance to the leading edge position can beincreased and a possibility that erroneous reading of important data of,for example, address at the position immediately before the shift of apit train can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is an enlarged fragmentary plan view of a first embodimentof an optical recording medium according to the present invention.

[0039]FIG. 2 is a waveform diagram of reproduced signals from the mediumof FIG. 1.

[0040]FIG. 3 is a block diagram of an apparatus for recording andreproduction of the optical recording medium used in the presentinvention.

[0041]FIG. 4 is an enlarged fragmentary plan view of a second embodimentof the optical recording medium according to the present invention.

[0042]FIG. 5 is an enlarged fragmentary plan view of a third embodimentof the optical recording medium according to the present invention.

[0043]FIG. 6 is a similar view of a fourth embodiment of the opticalrecording medium according to the present invention.

[0044]FIG. 7 is a waveform diagram of reproduced signals from theoptical recording medium of FIG. 6.

[0045]FIG. 8 is an enlarged fragmentary plan view of a fifth embodimentof the optical recording medium according to the present invention.

[0046]FIG. 9 is a diagram showing an information structure in the fifthembodiment of the optical precording medium according to the presentinvention.

[0047]FIG. 10 is an enlarged fragmentary perspective view showing therelation between the prepit area and the groove in the fifth embodiment.

[0048]FIG. 11 is an enlarged fragmentary plan view showing details ofpositional displacement in the embodiments of the optical recordingmedium according to the present invention.

[0049]FIG. 12 is an enlarged fragmentary plan view of a sixth embodimentof the optical recording medium according to the present invention.

[0050]FIG. 13 is a diagram showing an example of a modulated code in thesixth embodiment of the optical recording medium according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Embodiment 1 (Optical Recording Medium)

[0052] Referring now to FIG. 1, there is illustrated, in enlargedfragmentary plan view form, an optical recording medium of the presentinvention. Grooves 84 each having a width of 0.6 μm and a depth of 50 nmand lands 85 each having a width of 0.6 μm are arranged alternately inthe radial direction of the medium and recorded marks 81 are formed onthe two kinds of areas. In other words, each of the groove 84 and land85 serves as a recording area. In a prepit area 83, any groove is notformed but pits 82 are disposed on an extension of the boundary betweena land and a groove. Each of the pits has a width of 0.35 μm and a depthof 50 nm. The prepit area is divided into a first prepit area 831 and asecond prepit area 832. In the first prepit region 831, pits 82 aredisposed on the upper side, as viewed in the drawing, of the center lineof the land 85 and in the second prepit area 832, pits 82 are disposedon the lower side, as viewed in the drawing, of the center line of theland 85. Accordingly, when an optical spot 21 scans, for example, theland 85, pits on only either one of the sides are always reproduced andthere is no fear that crosstalk will occur between adjacent tracks.Therefore, address data recorded in the form of the prepits can duly bereproduced without crosstalk.

[0053] Since the pits 82 do not adjoin to each other in the radialdirection, they can be formed with ease. Also, pits 82 are uniformlydisposed on both sides of a track (a land or a groove) and hence theinfluence on a tracking servo signal, which is caused by the pits 82,can be canceled. Accordingly, the tracking offset can be suppressed to asufficiently small level.

[0054] Further, when reproducing, for example, a land 85, reproductionof address data at the second prepit area 832 is carried outcontinuously with reproduction of address data at the first prepit area831. Accordingly, when the two areas are united into one area in whichinformation is arranged to provide address data for one track, anaddress (track number) of a land and that of a groove can be setindependently of each other. In other words, by sequentially reproducingthe address data pieces in the first and second prepit areas 831 and832, discrimination between the land and the groove can be ensured.

[0055] More particularly, for reproduction of the groove, address datarepresented by prepits arranged in the first prepit area is made to beidentical to that represented by prepits arranged in the second prepitarea but for reproduction of the land, address data represented byprepits in the first prepit area is made to be different from thatrepresented by prepits in the second area. When addresses represented byprepits in the first and second prepit areas are different from eachother, a correlation may be set up between the two addresses and theefficiency of error correction code can be increased by utilizing thecorrelation.

[0056] Preferably, synchronous information (VFO) 86 and address data 87may both be arranged in each of the first and second prepit areas.

[0057] While in this example the prepit area is divided into two of thefirst and second prepit areas, the number of division which is pluralmay suffice. For example, when the number of division is four, pits infirst and third prepit areas may be arranged on one side of a groove andpits in the second and fourth prepit areas may be arranged on the otherside of the groove. By increasing the number of division of the prepitarea, reliability against, for example, defects can be improved.

[0058] Here, a phase change recording material (GeSbTe) is used for therecording film. Accordingly, the recorded mark is formed in the form ofan amorphous domain.

[0059] Referring now to FIG. 3, there is illustrated an example of aconfiguration in which the optical recording medium of the presentinvention is applied to an optical recording/reproducing apparatus. Inthe apparatus, a semiconductor laser 311 having a wavelength of 680 nmand a collimating lens 312 are used as a light source 31. A beam profileformer such as a prism may also be provided as necessary. Power of thesemiconductor laser is controlled by a light power controller 71 havingthe auto-light-power-control function. A light beam 22 emitted from thelight source 31 is focused on a magneto-optical recording medium 8 bymeans of a focusing optics 32. The focusing optics 32 has at least onelens 321 and in this example, it also has a beam splitter 324. Anobjective lens 321 for focusing the light beam on the optical recordingmedium 8 is designed to have a numerical aperture of 0.6. Therefore, anoptical spot 21 on the optical recording medium 8 has a diameter of 1.0μm. The optical spot can be moved to a desired position on the opticalrecording medium 8 by means of a scanning unit 6. The scanning unit 6includes at least a motor 62 for rotating the disc-like magneto-opticalrecording medium 8 and an auto-position controller 61 having thefunction of auto-focus control and auto-tracking. The auto-positioncontroller 61 utilizes a reflected beam 23 from the magneto-opticalrecording medium 8 to cause a photodetection unit 33 to detect anoptical spot position which is used for feedback control.

[0060] The optical spot position can be detected by detecting power of adiffracted light ray from a groove. The photodetection unit 33 isconstructed of a lens, a beam splitter and a plurality ofphotodetectors, and output signals of the plurality of photodetectorsare calculated to produce a servo signal and a reproduced signal.

[0061] With the optical recording medium as shown in FIG. 1 used,signals as designated at 14 in FIG. 2 are produced as prepit signals.The signal is inputted to an address detection unit to decode addressdata and at the same time, timings of signals of the first and secondprepit areas are detected and on the basis of the timing information,the amplitude (averaged peak-to-peak amplitude) of the first prepit areaand that of the second prepit area are stored. The thus storedamplitudes are compared with each other by means of an amplitudecomparator to produce tracking offset information which in turn is fedback to the position moving unit (scanning unit). Referring to FIG. 2,when the optical spot scans a groove, a magneto-optical reproductionsignal 11 and a corresponding prepit signal 14 (an upper one in thedrawing) are produced and when the optical spot scans a land, amagneto-optical reproduction signal 12 and a corresponding prepit signal14 (a lower one in the drawing) are produced. Since in this example theoptical spot is slightly offset as shown in FIG. 1, an amplitudedifference 13 takes place between a prepit signal from the first prepitarea 831 and a prepit signal from the second prepit area 832. Thisamplitude difference corresponds to a tracking offset amount.

[0062] By using the apparatus of FIG. 3, the tracking offset could bereduced to ±0.03 μm or less even when various kinds of externaldisturbance such as aberration of the optical spot are taken intoconsideration. Under the nominal state devoid of optical aberration, thetracking offset was ±0.015 μm or less.

[0063] As described above, in the present invention, prepits aredisposed on both sides of a virtual extension of the center line of agroove or a land as shown in FIG. 1. Consequently, offset is reduced tomake tracking offset hardly occur. Since prepits do not exist on bothsides of any specific position of the center line of a groove or a land,interference of prepit information between adjacent tracks does not takeplace within a reproduced spot and hence high-density narrow trackrecording can be ensured.

[0064] Further, if a tracking offset occurs as shown in FIG. 2, thetracking offset amount can be detected accurately by comparingamplitudes of signals of prepits located on both sides. Accordingly, byfeedback-controlling the information indicative of a comparison resultto the scanning unit, the tracking offset can be suppressed.

[0065] Furthermore, discrimination between the groove and the land canbe effected with ease.

[0066] By using the optical recording medium of the present invention,the tracking offset can be suppressed to a practically sufficientlysmall level (0.03 μm or less) and besides, address data can easily beobtained even during high-density narrow track recording. Through theuse of the optical recording/reproducing apparatus of the presentinvention, the tracking offset can readily be reduced by feedbackcontrol.

[0067] Embodiment 2

[0068] Referring now to FIG. 4, there is illustrated a second embodimentof the present invention. A medium of the present embodiment differsfrom that of embodiment 1 in that only synchronous information pits 861to 864 are disposed on the upper side (as viewed in the drawing) of thecenter line of a groove 841, 842, 843, 844 or 845 and synchronousinformation pits 861 to 864 and address data pits 871 to 874 are bothdisposed on the lower side (as viewed in the drawing) of the center lineof each of the grooves 84. Preferably, the address data pits 871 to 874are arranged continuously to the synchronous information pits 861 to864. For a land 85, the upper and lower side relation is inverted.

[0069] Being different from the embodiment 1, the present embodiment hasaddress data arranged on only the upper or lower side of the center lineof the groove or the land and therefore the same address data isallotted to the land and groove. In the present embodiment, fourdivisional prepits areas 831 to 834 are provided with the aim ofimproving the reliability of the prepit area but the prepit area is notalways divided. In the present embodiment, the synchronous pit 861 inthe first prepit area 831 are designed to have a longer length than thesynchronous pits in the second to fourth prepit areas by taking intoaccount the influence of aliasing of a signal which has passed through alow pass filter. Preferably, pits disposed on the upper and lower sidesare spaced apart from each other by 0.5 μm or more from the viewpoint offabrication of the medium. More preferably, they are spaced apart by adistance of about 1 μm which is the diameter of the reproduced opticaldisc spot.

[0070] Embodiment 3

[0071] Referring to FIG. 5, there is illustrated a third embodiment inwhich identification marks 88 are used to discriminate the land from thegroove. In the present embodiment, identification marks 88 fordiscrimination between the land and the groove are providedindependently of the prepit area in the embodiments 1 and 2.

[0072] In the present embodiment, a pair of pits (identification marks)88 are arranged on the upper and lower (as viewed in the drawing) sidesof the center line of a groove 841, 843 or 845 but they are not providedfor a groove 842 or 844. On the assumption that an optical spotrelatively moves from left to right as viewed in the drawing when themedium provided with the above identification marks is reproduced toprovide a case of “presence” where the identification marks are seen bythe optical spot and a case of “absence” where the identification marksare not seen, “presence, presence” is held for the groove 841, “absence,presence” is held for a land 851, “absence, absence” is held for thegroove 842 and “presence, absence” is held for a land 852. Further,“presence, presence” is held for the groove 843, “absence, presence” isheld for a land 853, “absence, absence” is held for the groove 844 and“presence, absence” is held for a land 854. Namely, either one of“presence, presence” and “absence, absence” is held for the groove andeither one of “absence, presence” and “presence, absence” is held forthe land. Accordingly, this can be utilized to effect discriminationbetween the land and groove on the basis of a reproduced signal. Tosecure reliability, a plurality of pairs of identification marks maypreferably be provided and more preferably, the paired pits are spacedapart from each other by several μm or more in the circumferentialdirection or information track direction of the medium which isperpendicular to the radial direction. For example, the prepit area inthe foregoing embodiments and the identification mark area maypreferably be arranged alternately in the circumferential direction.

[0073] Embodiment 4

[0074] Referring to FIG. 6, there is illustrated, in enlargedfragmentary plan view form, an optical recording medium according to afourth embodiment of the present invention. Grooves 84 each having awidth of 0.5 μm and a depth of 40 nm and lands 85 each having a width of0.5 μm are arranged alternately and recorded marks 81 are formed on thetwo kinds of areas. In other words, each of the land 85 and groove 84serves as a recording area. In a prepit region 83, any groove is notformed but substantially circular pits 82 (each having a diameter of 0.3μm and a depth of 40 nm) are disposed on an extension of the boundarybetween a land and a groove. The prepit area is divided into a VFO(variable frequency oscillator) area 833 and an address area 834.Especially, in the VFO area, pits 82 are disposed alternately on theupper and lower sides of the center line of a land 85. In the addressarea, pits 82 are disposed alternately at the same period as that in theVFO area. Accordingly, there are no pits which exist on both sides of aposition of the center line of the land or the groove.

[0075] In addition, in the address area, data for a particular track isso encoded as to differ by one pit from data for an adjacent track. Inother words, the data takes the form of a Gray code. With thisconstruction, when an optical spot 21 scans, for example, a land 85,only pits on either one side are always reproduced and there is no fearthat crosstalk will occur between the adjacent tracks. Therefore,address data distributed to the prepits can duly be reproduced withoutcrosstalk. Since pits 82 for adjacent tracks do not adjoin to eachother, they can therefore be formed with ease. Also, pits 82 areuniformly disposed on both sides of a track (a land or a groove) andhence the influence on a tracking servo signal which is caused by thepits 82 can be canceled. Accordingly, tracking offset can be suppressedto a minimum.

[0076] When the medium of the FIG. 6 embodiment is reproduced with theapparatus of FIG. 3, reproduced signals as shown in FIG. 7 are generatedfrom the prepit area 83, indicating that data pieces which differ trackby track can be obtained and therefore address data is recorded veryhighly efficiently. Thanks to the use of the Gray code, an address canbe reproduced even in the course of inter-track access, ensuringsuitability to high-speed access. Further, with the Gray code used, anerror hardly occurs even in the presence of crosstalk, thus ensuringsuitability to narrowing of tracks.

[0077] Embodiment 5

[0078] Referring now to FIG. 8, there is illustrated in enlargedfragmentary plan view form an optical recording medium according to afifth embodiment 5 of the present invention. Groove 84 each having awidth of 0.7 μm and a depth of 70 nm and lands 85 each having a width of0.7 μm are arranged alternately in the radial direction and the twokinds of areas serve as information tracks on which recorded marks canbe formed.

[0079] In other words, each of the land 85 and groove 84 serves as arecording area. In a prepit region 83, any groove is not formed but pits82 are disposed on an extension of the boundary between the land and thegroove. The prepit area is divided into zones which are arranged in theradial direction over about 1800 information tracks, that is, 900grooves.

[0080] The zones are arranged concentrically of the whole of a disc insuch a manner that 24 zones in total are in a disc having a radius of 30to 60 mm. More specifically, in each zone, the number of prepit areas tobe detected during one revolution, that is, the sector number isconstant and the sector number is larger in an outer zone than in aninner zone.

[0081] An example of structure of each sector 41 is shown in FIG. 9. Thesector 41 has a prepit area 83 at the head of a data recording area.

[0082] As shown in FIG. 8, the prepit area is divided into a firstprepit area 831 and a second prepit area 832. In the first prepit area831, pits 82 are arranged on the upper side (as viewed in the drawing)of the center line of a land 85 and in the second prepit area 832, pits82 are arranged on the lower side of the center line of the land 85.Consequently, for example, when an optical spot 21 scans the land 85,only pits on either one side are always reproduced and there is no fearthat crosstalk will occur between adjacent tracks. Accordingly, addressdata allotted to the prepits can duly be reproduced without crosstalk.Address data represented by the prepits is recorded in the form of a 1-7modulation code (having a channel bit length of 0.2 μm). In other words,the linear recording density is 0.3 μm/bit.

[0083] The relation between the prepit area and the groove in thepresent embodiment is illustrated in enlarged fragmentary sectionperspective view form in FIG. 10.

[0084] In the present embodiment, a gap area 87 is provided between thefirst and second prepit areas 831 and 832 to space them apart by about1.0 μm. Since in this embodiment data is recorded pursuant to the 1-7recording, the gap distance corresponds to a length of about 5 channelbits. The 5 channel bit length is exactly the middle length between thelongest mark length (8 channel bit length) and the shortest mark length(2 channel bit length). Therefore, the gap area between the first andsecond prepit areas can be reproduced having a length which lies betweenthe shortest mark length and the longest mark length even when the pitsundergo changes in shape and position during formation of the pits andthe optical spot undergoes a change in shape and a change in scanningposition (servo offset), thus ensuring very high reliability. In thisexample, the marks are designed to undergo, at the worst, a total changein position which is suppressed to 0.6 μm (3 channel bit length) andtherefore, the effective length (during reproduction) is 2 channel bitsin the case of the shortest length and 8 channel bits in the case of thelongest length to match the rule of the 1-7 modulation code, thusraising no problem during reproduction. If the detection length islonger than 8 channel bit length, then it will adversely interfere witha special synchronous pattern such as a recorded address mark. If thedetection length is shorter than 2 channel bits, then a small markresults which is less than resolution of the reproduction optical spotand cannot be detected. Accordingly, it is preferable that the gaplength be suppressed to the middle between the longest mark length andthe shortest mark length as in the present embodiment.

[0085] Depending on the specification of a pit forming apparatus, thechange in mark position can be suppressed to one channel bit length orless. In this case, the nominal gap length may be suppressed to 3 to 7channel bit length but the pit forming apparatus for this purposebecomes expensive. There is a high possibility that signals suffer anerror attributable to a tracking offset during reproduction andtherefore the medium is desired in which preferably, the gap length isexactly the middle between the longest mark length and the shortest marklength permissible for the recording as described hereinbefore.

[0086] In the present embodiment, pits 82 are uniformly disposed on bothsides of the center line of a track (a land or a groove) and hence theinfluence on a tracking servo signal which is caused by the pits 82 canbe canceled. Accordingly, the tracking offset can be suppressed to asufficiently small level. In addition, for example, when a land 85 isreproduced, reproduction of address data at the second prepit area 832is carried out continuously with reproduction of address data at thefirst prepit area region 831. Accordingly, when the two areas are unitedinto one area in which information is arranged to provide address datafor one track, an address (track number) of a land and that of a groovecan be set independently of each other. In other words, by sequentiallyreproducing the address data pieces in the first and second prepit areas831 and 832, discrimination between the land and the groove can beensured.

[0087] More particularly, for reproduction of the groove, address datarepresented by prepits arranged in the first prepit area is made to beidentical to that represented by prepits arranged in the second prepitarea but for reproduction of the land, address data represented byprepits in the first prepit area is made to be different from thatrepresented by prepits in the second area. When addresses represented byprepits in the first and second prepit areas are different from eachother, a correlation may be set up between the two addresses and theefficiency of error correction code can be increased by utilizing thecorrelation.

[0088] Preferably, synchronous information (VFO) 86 and address data 87may both be arranged in each of the first and second prepit regions.

[0089] While in this example the prepit area is divided into two of thefirst and second prepit areas, the number of division which is pluralmay suffice. For example, when the number of division is four as shownin FIG. 5, pits in the first and third prepit areas may be arranged onone side of a groove and pits in the second and fourth prepit areas maybe arranged on the other side of the groove. By increasing the number ofdivision of the prepit area, reliability against, for example, defectscan be improved.

[0090] Here, a phase change recording material (GeSbTe) is used for therecording film. Accordingly, the recorded mark is formed in the form ofan amorphous domain.

[0091] Referring now to FIG. 11, amounts of positional displacement 963between prepit areas of adjacent tracks, 961 between prepits of adjacenttracks and 962 between grooves of adjacent tracks in the medium areillustrated in greater detail. In the actual medium, positionaldisplacement sometimes occurs between pits of adjacent tracks owing tovarious causes taking place during pit formation. Because of thepositional displacement amounts 961, 962 and 963, the length of gapareas 86 and 87 is increased or decreased.

[0092] In addition to the above positional displacement, various kindsof variations (aberration, servo error and the like) during reproductionalso cause apparent positional displacement of reproduced signals.Accordingly, the positional displacement possibly leads to a seriousproblem. But in the present invention, the nominal length of the gaparea is set to the middle length between the shortest mark length andthe longest mark length pursuant to the 1-7 modulation code and hence apositional displacement amount of ±0.6 μm is permissible.

[0093] The optical recording medium shown in FIG. 8 can be reproducedwith the apparatus shown in FIG. 3 in a similar manner to that describedin connection with embodiment 1, bringing about advantages that trackingoffset can be reduced to ±0.03 μm or less even when various kinds ofexternal disturbance such as optical aberration are taken into accountand in particular, it can be reduced to ±0.015 μm or less under thenominal state devoid of optical aberration.

[0094] Embodiment 6

[0095] While the embodiment of FIG. 8 uses the 1-7 modulation coding asthe recording modulation coding, the present embodiment uses eight tofourteen modulation (EFM) recording. The channel bit length is about 0.2μm. In the present recording, the shortest mark length is 3 channel bitlength and the longest mark length is 11 channel bit length.Practically, a mark having a length of 12 channel bits or more isavailable but this type of mark is limited to a special application suchas a synchronous pattern. Accordingly, data must avoid inclusion of apattern which may possibly interfere with the special pattern. Theprepit area, groove and land are arranged similarly to the embodiment 5of FIG. 8 excepting points to be described later. Namely, they arearranged as shown in FIG. 10 and especially, each groove and each have awidth of about 0.75 μm and each groove and each prepit have a depth ofabout 0.075 μm.

[0096] In the present embodiment, the prepit area and the groove aredisposed as shown in FIG. 12. Four prepit areas 831, 832, 833 and 834are allotted to the head of one sector. In each prepit area, a VFO areafor synchronization to reproduced signals and an address area recordingaddress data of the track and sector are arranged sequentially. Startpositions of pits as well as end positions are so arranged as to besubstantially aligned in the radial direction and a gap area 86 isprovided between the end of the groove and the start of the pit area.Likewise, a gap area 87, 88 or 89 is provided between adjacent prepitareas.

[0097] As described previously, details of positional displacementbetween the start position of a pit and the start position of asucceeding pit is depicted in FIG. 11. With the displacement as shown inFIG. 11, the effective length of the gap area 86 is decreased orincreased as in the embodiment 5 of FIG. 8. In order to align ends offinal pits of the prepit areas 831, 832 and 833 in the radial direction,an additional pit pattern 110 as shown in FIG. 13 is used. Theadditional pattern selected from four types (a), (b), (c) and (d) inaccordance with the preceding data is used. Through this, trailing edgepositions 99 of the final pits can always be aligned to the sameposition regardless of the preceding data and the gap length and pitlength can be limited to the lengths allowed for the modulation code. Inthis manner, the gap area between the succeeding prepit area 120 and thetrailing edge position 99 of the final pit can be set to the middlelength between the longest mark length and the shortest mark lengthpursuant to the modulation coding, so that the margin can be greatlyincreased during prepit formation and reproduction as in the embodiment5.

[0098] In the foregoing embodiments, the medium of the phase changerecording material is described but it may be of another material toattain the advantages of the present invention. For example, amagneto-optical recording film may be used as the recording film. Inaddition, the modulation code has been described as being of 2-7 and 8/9coding but it may be of another type in which the previously describedEFM is extended.

[0099] According to the present invention, in the optical recordingmedium having the lands and grooves, the substrate can be fabricatedwith a simple mastering apparatus and replica can also be prepared withease, with the result that the medium fabrication margin and readoutmargin which are practically sufficiently large can be ensured.Accordingly, a cheap and high-density optical recording medium can beprovided.

1. An optical recording medium having an aligned prepit portionstraddled on a plurality of tracks in a radial direction; wherein theprepit portion includes first and second prepit portions divided in atrack direction; wherein a prepit of the respective first and secondprepit portions is arranged on a boundary of the respective tracks;wherein the first prepit portion has an address information prepit and asynchronous information prepit; wherein the second prepit has asynchronous information prepit; wherein the prepit of the first prepitportion and the prepit of the second prepit portion are arranged atevery two-track pitch in the radial direction; wherein the prepit of thefirst prepit portion and the prepit of the second prepit portion arearranged with one track displaced in the radial direction; and wherein agap is provided between the first and second prepit portions.
 2. Anoptical recording medium according to claim 1, wherein atrack-directional length of the gap is at least equal to a shortest-marklength determined by a recording system.
 3. An optical recording mediumaccording to claim 1, wherein a track-directional length of the gap isno greater than a largest-mark length determined by a recording system.4. An optical recording medium according to claim 1, wherein atrack-directional length of the gap is smaller than a largest-marklength by at least one channel-bit and larger than a shortest-marklength by at least one channel-bit.
 5. An optical recording mediumaccording to claim 1, wherein the track-directional length of the gap is½ of a total of a largest-mark length and a shortest-mark length.